| Determination of SAR values when using mobile phones with headsets |
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Topic Determination of SAR values when using mobile phones with headsets Start 15.06.2007 End 31.01.2008 Project Management IT'IS Foundation, Zurich, Switzerland Objective Wired and wireless headsets for use with mobile phones had to be compared with maximum SAR10g values in the user’s head. The project aimed at answering the question whether using a wired headset always results (i. e. irrespective of frequency band and cable routing) in lower SAR10g values compared to exposures caused by wireless devices (usually operating according to the Bluetooth standard). Additionally the influence of the cable routing on SAR10g values in the user’s trunk had to be analyzed. The results will be used for specifying recommendations on precautionary measures regarding the reduction of exposure to high frequency electromagnetic fields when using mobile phones. Interim Results SAR has been estimated by means of measurements and numerical computations in the course of this project. For this purpose different homogenous head and flat phantoms were used for measurements and two different anatomical whole body models for computer simulations. Findings have been compared to SAR values resulting from the mobile phone operated directly (without a headset) at the user’s head and/or at the user’s trunk respectively. Two different mobile phones (one “flip-phone” with external stub antenna, one “candybar”-type phone with integrated antenna), three wired and three wireless headsets were investigated by means of measurements. Additionally one mobile phone was analyzed together with a wired headset by means of computer simulations. In scenarios involving exposure of the user’s trunk different orientations of the mobile phone (front, back facing the body) and distances to the trunk have been considered. “Tilt” and “touch” position, well known from compliance testing, were used for estimating the SAR in the user’s head by means of measurements, corresponding to situations when the mobile phone is operated directly at the user’s ear or head. All tests were performed for voice communication only (no data transmission modes), utilizing GSM 900, GSM 1800 and UMTS 1950 systems. The results can be summarized as follows: SAR caused by wireless headsets has been found very low (SAR10g < 0,005 W/kg). Almost all measurements taken did not exceed the noise level of the measurement setup. Dependent on the cable routing in the vicinity of the mobile phone’s antenna, a certain amount of radiated RF power can couple to the cable of a wired headset, resulting in SAR values in the user’s trunk along the cable routing and in the user's head finally. SAR10g-values in the user’s head were found to be lower for headsets with two earpieces because the power splits and thus results in reduced SAR. No GSM 900 scenario with wired headsets resulted in higher SAR-values in the user’s head compared to the overall maximum SAR, which can be expected when the mobile phone is operated directly at the user’s ear (cable routing chosen so that strongest coupling effects result; possible influence of the user’s hand holding the phone against the head [in scenarios without headsets] and dynamic power regulation [in scenarios with and without headset] neglected). With UMTS and especially with GSM 1800, however, configurations could be identified, which result in an up to 2,5 fold increase of maximum SAR10g in the user’s head when using a wired headset compared to the maximum value found when operating the respective mobile phone directly at the user’s ear (using the same/identical mobile phone system respectively frequency band). With respect to the GSM 1800 system this effect was found for all three wired headsets and for both mobile phones being investigated. With respect to UMTS the effect occurred only with one mobile phone in combination with two of the three headsets and the effect was less distinct (increase only by a factor of 1,3). An increase in SAR was found when the cable of the headset was not routed in close proximity of the human body. Situations like that can occur when the mobile phone is placed onto a desk during phone calls. As the maximum (measured) SAR10g-values in the head-phantom are comparably low in situations when the mobile phones are operated in the GSM 1800 and/or UMTS-band in standard positions at the user’s ear, the reported increase by a factor of 2,5 does not result in any violation of exposure limits. Similar to GSM 900, routing the cable in close proximity to the trunk results in reduced maximum SAR-values in the user’s head for GSM 1800 and UMTS, too. Depending on the possible combinations of headsets and mobile phones and depending also on the specific cable routing reductions by a factor of 1,2 to 372 were found. For GSM 900 the factors were found to range from 2,8 to 251. In this project the overall maximum SAR10g-value measured in the vicinity of a wired headset’s earplug was found to be 0,33 W/kg. Simulations by means of numerical computations using anatomical correct models of two different human heads confirm the order of magnitude (0,37 W/kg). Operating mobile phones at the trunk of the human body result in quite different SAR10g values (in the trunk) depending on the orientation of the mobile to the body surface. Differences up to 15 fold have been found. The overall maximum value was measured in a situation, when the back of the “candybar” mobile was placed against the flat phantom. With 3,2 W/kg the exposure exceeds the recommended limit of 2,0 W/kg considerably. Additionally the exposure depends clearly on the distance between the mobile phone and the body surface, as expected. Connecting a wired headset to a mobile phone which is operated at the trunk of the body alters the maximum SAR10g values in the trunk only to a minor degree in most cases. This holds true even for worst-case coupling conditions with respect to wire routing in the vicinity of the mobile phone’s antenna. When the mobile phone is orientated with its back to the body (phantom), a small decrease in maximum SAR, otherwise a small increase was observed. Measurements and computational simulations are in good agreement concerning this general observation. Different behavior was recorded in comparison of the two anatomical body models, with respect to the transmitting frequency (900/1800/1950 MHz) and with respect to the exact position of the mobile phone (“pant pocket” or “shirt pocket” position). No general statements can be derived from the data being presented as results of these numerical studies. In one case (UMTS, front of the mobile phone facing the body surface, whole-body model “Billie”) the maximum SAR value in the trunk increased from 0,5 W/kg to 1,1 W/kg, while it stayed almost constant at 0,7 W/kg with the other model (“Duke”). The final report is available as a PDF-file(4,076 kb). Conclusions The exposure of the head can be reduced significantly by using headsets for mobile phones. Wireless headsets (maximum Bluetooth class II) result in quite low exposures, irrespective of the mobile phone system (GSM 900, GSM 1800 or UMTS) and also irrespective of the mobile phone’s dynamic power control. Compared to situations when the mobile phone is operated at the user’s ear with direct contact to the head, using wired headsets may result in higher local exposures of the user’s head in terms of SAR10g in the same frequency band, if disadvantageous conditions are taken into account. In the course of this project the effect of increasing SAR could be seen with GSM 1800 and UMTS only, but not with GSM 900. Therefore, the effect strongly depends on the frequency band being used and especially on the cable routing, starting from the mobile phone along the trunk towards the user’s head and ear(s). In scenarios with strong coupling of RF-power to the headset cable, an increase of SAR in the user's head may result if the cable is not routed in close proximity to the body. Nevertheless, in the predominant number of possible configurations a substantial reduction of exposure can be achieved by means of wired headsets, too. When the mobile phone is operated at the user's trunk, exposure may exceed the recommended limit of 2,0 W/kg, if disadvantageous or worst-case conditions are taken into account. Therefore, attention should be paid to any safety notes with respect to operating the mobile phone at the trunk of the body, which may be written down by the manufacturer e. g. in the mobile phone's user manual. Manufacturers should take technical measures to prevent RF power from coupling to the headset cable and/or to prevent it from being conducted along the cable up to the user's head. References
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